The present invention pertains to apparatus and methods for cleaning gases, including gases contaminated by particulate and/or gaseous contaminants. In the arts concerned with gas cleaning there are numerous disclosures of gas scrubber apparatus and methods in which charged or uncharged liquid droplets are injected into a flowing stream of contaminated gas, with an injection velocity greater than the gas flow velocity, so that contaminant particles or gases may be removed by the scrubbing effect of the droplets on the contaminated gas, and in which the droplets carrying absorbed contaminants are then removed from the gas stream by various forms of demisting apparatus or methods, so as to remove contaminants from the gas stream, to the extent of the scrubbing efficiency of the apparatus or method.
Examples of such apparatus and methods involving use of charged liquid scrubbing droplets are disclosed in applicant's prior U.S. Pat. Nos. 6,156,098; 6,551,382; and 6,986,803.
The efficiency of a gas scrubber in removing contaminants from the gas stream clearly depends strongly on the total number of collisions between liquid droplets and the gas molecules or contaminant particulates. The number of collisions of course depends both on the liquid droplet flow rate and the path length of the motion of the droplets through the gas. One obvious way to increase overall gas scrubber efficiency would be to increase the liquid droplet flow rate. This approach is suggested in prior art, e.g. in two of applicant's above-referenced patents, pointing out the dependence of collection efficiency on the number of liquid droplets, U.S. Pat. No. 6,156,098, col. 2, lines 25-32; U.S. Pat. No. 6,551,382, col. 1, lines 40-45. Another obvious manner of increasing collection efficiency would be to increase the path length for droplet motion through the gas by increasing the size of the gas scrubber apparatus. This approach is suggested by applicant's U.S. Pat. No. 6,156,098 at col. 10, lines 54-57, pointing out the dependence of the gas volume swept out per liquid droplet on the average length of travel of the droplets through the gas. But this approach is of limited practical value, because multiple stages would be required to achieve a significant path length increase, since in a given stage the higher speed droplets quickly decelerate, due to drag from droplet/gas molecule collisions, while those collisions also accelerate the gas velocity, so that the droplets and gas quickly attain the same speed, with zero relative speed, and multiple stages would involve greater complexity and cost for the scrubber apparatus.
In the course of his work with gas scrubbers used in varying applications and environments, applicant has realized a need for improving gas scrubber efficiency without having to use either a higher droplet flow rate or a larger scrubber apparatus. There are applications for gas scrubbers, for example shipboard applications, in which limitations of available scrubber space and liquid storage space may preclude effective use of either of these approaches. One application for smaller scrubbers would be in engine exhaust cleaning apparatus for ferries.
Applicant's present invention allows a significant increase of gas scrubber efficiency in an unexpected manner, without employing either an increased droplet flow rate or a larger gas scrubber apparatus. This unexpected result is achieved through a synergistic interaction of the liquid droplet injection, with an effect of a pressure-drop plate downstream from the droplet injection site, which pressure-drop plate has an adjustable gas flow constriction causing an adjustable pressure drop across the pressure-drop plate, resulting in an adjustable gradient of increasing gas pressure in the downstream direction from the droplet injection site to the pressure-drop plate.
As already noted, the liquid droplets are injected with an injection velocity greater than the gas flow velocity. The path length for droplet interaction with gas, is the path length for motion of the droplets relative to the gas. This path length will be affected by the relative velocities of the droplets and the gas.
For typical gas scrubbers, the initial liquid droplet spray momentum flux is about an order of magnitude greater than the initial gas momentum flux (momentum flux being droplet or gas momentum entering the scrubber per unit time and per unit cross sectional area of the scrubber).
Due to viscous drag forces on the droplets decelerating the droplets, and momentum transfer from the droplets to the gas, the liquid droplet momentum flux will accelerate the gas velocity, so as to decrease the relative velocity of the droplets and gas. In the absence of the pressure-drop plate and its resulting pressure gradient, the relative velocity of the droplets and gas will decrease rapidly across the scrubbing region, thus limiting the effective relative droplet-gas motion path length for droplet-gas interaction.
But with the presence of the pressure-drop plate, the gradient of increasing pressure in the gas flow direction will decelerate the gas flow velocity, already less than the droplet velocity, so as to increase the relative velocity of the droplets and the gas, and thus increase the effective relative droplet-gas motion path length, resulting in increased collection efficiency due simply to the pressure gradient effect on relative droplet and gas motion. As detailed below, reasonable values of other operating scrubber parameters allow collection efficiency increase from 70% to 99%, due simply to the adjustable pressure gradient effect, without any need to increase the liquid droplet flow rate or increase the size of the gas scrubber.
It is not the intent of this application, by stating that certain embodiments of the present invention are suited to certain purposes or to dealing with certain problems, to necessarily limit the scope of the invention to only embodiments which are useful for said purposes or problems; it is instead the intent that the scope of the invention be determined by the claims as more fully stated below.